Tuning devices for receivers in communication service use are used to tune the receiver to a certain frequency, the tuned-in station being usually indicated on a scale.
Digital tuners have been designed according to the principle of digitization of a tuning voltage. Such systems have very high requirements with respect to constancy of the reference voltage or of a digital-to-analog converter therein as well as the voltage-to-frequency converter, respectively.
It has also been proposed to use a frequency synthesizer.
A synthesizer is a circuit arrangement capable of providing a great number of discrete frequencies all of which are derived from a highly stable oscillator.
A phase-locked loop quartz digital tuner (see, for example, "Funkschau" 1976, number 20, pages 839-843) operating on the synthesizer principle has been published. In this tuner the frequency from the oscillator is fed via a programmable frequency divider to a stage serving as a frequency and phase locked loop (PLL), and compared in the PLL with a quartz-controlled reference frequency of 25 kc/s. A correction or error pulse sequence in form of a duty factor variation is developed during the aforementioned process; the average voltage with respect to time is developed in an integrator and supplied to both the oscillator disposed in the receiving section and the r-f stage so as to serve as the tuning voltage.
This known arrangement has some important disadvantages. The frequency of the tuner is not continuously variable. The selectable nominal incoming frequencies are to be found in a frequency pattern of 100 kc/s. The tuning pattern is too coarse to be used to tune amplitude-modulated stations.